Noise limiter



Sept. 22, 1942.

NOISE LIMITER 4 sheets-sheet 1 Filed March 16, 1940 Sept. 22, 1942.

c. c. MARTINELLI v 2,296,393

NOISE LIMITER Filed March 16, 1940 4 Sheets-Sheet 3 .sr/vc. GEH l Z/VU.E?! y0-DE L/MNL'H moms' Ffm. f

VIDEO HMP.

nventor I Cz/ro. @.fartinelli Cfttorneg swf 22, 1942 c. c. MARTINELU2,296,393

NOISE LIMITER Filed March 16, 1940 4 Sheets-Sheet 4 F1 +200 i I G" Z E 7E 1'0 cnw/005 )wir TUBE IIG. 8.

10.5' 'rovi/1050 HMP.

5WD 'sy/vc. SEP/wwwa "IIN" Bnvenfor Ciro C. Martin ZZIL (IttomegPatented Sept. 22, 1942 0151111511) STATES- PATENT OFFICE Y 2,296,393

Norse LIMITEE Ciro C. Martinelli, Oaklyn, N. J., assignor to RadioCorporation of' America,V a corporation o! Delaware 4Claims.

My invention relates to limiting or clipping circuits for radioreceivers and, in its preferred form, particularly to noise limiters fortelevision receivers.

In television receivers it may be desirable to employ limiting circuitsfor various purposes, such as for example, leveling synchronizing pulsesbefore they are impressed upon `the deflecting circuits, or reducing theamount of noise in the received signal. Different circuits are known atthe present time which will perform these functions, but many of themare not altogether satisfactory for one reason or another.

It is, accordingly, an object of my invention to provide an improvedlimiting circuit which will be effective in operation and simple indesign and adjustment.

A further object of my invention is to provide an improved means for andmethodof limiting noise in a radio receiver, either for sound or picturereception.

A further object of my invention is to provide an improved means for andmethod of leveling synchronizing pulses in a television receiver.

A still further object of my invention is to provide an improved meansfor and methodof effecting gain control in a television receiver.

In a preferred embodiment of my invention the limiter circuit comprisesa diode and an output resistor in series with each other and also in 30series with a direct current voltage which normally maintains a flow ofcurrent throughsaid diode and resistor. The picture signal, synchronizing pulses, or the like, are impressed upon the limiter circuitwhereby they cause a variation in 35 the diode current to produce asignal voltage across the output resistor, and they are impressed uponthis circuit with such polarity that an increase in signal amplitudebeyond a certain limit causes the flow of diode current to stop. Thus,40

Figure 1a is a group of curves which are re- 50 ferred to in explainingthe operation of the cir. cuit of Fig. 1,

Figure 2 is a circuit diagram illustrating an= other embodiment of myinvention,

Figure 2a is a group of curves which are re- 55 ary coil I2 and thelarge capacity condenser I3 ferred to explaining the operation of thecircuit of Fig. 2,

Figure 3 is a circuit diagram of an embodi-A ment of my invention asapplied to an automatic volume control circuit of the type operating onthe peaks of synchronizing pulses,

Figure 4 is a circuit diagram of an embodiment of my invention asutilized to clipV or level synchronizing pulses,

Figure 4a is a group of curves that are referred to in explaining theoperation of the circuit of Fig. 4,

Figures 5 and 6 are circuit diagrams of other embodiments of myinvention as utilized to level synchronizing pulses,

Figure 7 is a circuit diagram of an embodiment of my invention in whichthe noise suppressor is' applied to the synchronizing channel only andFigures 8 and 9 are embodiments of the invention in which the limiterdiode current is prevented from flowing through the output resistor ofthe detector.

Referring to Fig. 1 the invention is shown applied to a televisionreceiver of the superheterodyne type comprising the usual radiofrequency and intermediate frequency amplifier portions (not shownexcept for the last yI. F. amplifier stage indicated at I), a seconddetector 5 comprising diodes 2 and 3 connected to form a full waverectifier and supplying signal through a noise suppressor diode 4 andits output resistor V6, a video amplifier tube," and a cathode ray tube8.

The part of the receiver which precedes the second detector 5 preferablyis designed in accordance with the teachings of my copending applicationSerial No. 287,027, filed July 28, 1939, and assigned to the RadioCorporation of America. Specifically, the time constants of the voltagesupply filters are made either very short or very long in order'toprevent a noise pulse from affecting the receiver for a period longerthan that of the noise pulse itself.

The particular coupling between the last I. F. amplifier tube i and thesecond detector 5, in the example illustrated, is of the type describedin the Grundmann and Allen Patent No. 2,157,170, issued May 9, 1939, andassigned to the Radio Corporation of America. 'I'he primary circuit,which includes a tunable primary coil 9, may be traced from the plate ofthe amplifier tube I through the primary coil 9, a blocking condenserII, and through a portion of the secondto ground. The secondary circuitcomprises the secondary coil I2 and the capacity of the diodes 2. and 3,this circuit being tunable like/the primary circult by means of an ironcore as indicated. A suitable damping resistor I4 preferably isconnected across the secondary coil I2.

The detector outpui-I resistor I6 is connected between the mid-point ofthe secondary coil I2 and, in this example,l the cathodes of the diodes2 and 3.

Instead of applying the picture signal and synchronizing pulses whichappear across the output resistor I6 directly to the video amplifier 1,they are rst passed through the limiter circuit comprising the diode 4and the output resistor 6. The diode 4 may be of the usual type havingan indirectly heated cathode I1 and a plate I8. In order to hold up thehigh frequency response the three inductance coils 2l, 22'and 23 may beemployed The inductance coil 2l 4corresponds to the usual peaking coilemployed in video amplifiers in series with the plate resistor while thecoils 22 and 23 may be referred to as series resonating coils since theyare caused to resonate with the capacities of the second detector diodes2 and 3 and the limiter diode 4, respectively. l

In accordance with the invention a flow of current is normallymaintained through the limiter diode 4 by applying a certain directcurrent potential thereacross. In the example illustrated in Fig. 1, theplate I8 of the diode .4 is connected to ground through the outputresistor 6, while the cathode I1 is connected through a resonating coil23 anda resistor 24 to a direct current voltage of negative polarity,this voltage being indicated on the drawings as the -17 volt point on avoltage/divider 20. Thus, -the plate-I8 of the limiter Adiode 4 ispositive a certain amount with respect toits cathode I1, and in theabsence of incoming signal there is a ow of current which may be tracedfrom the plate I8 throughthe diode 4, the coil 23, the resistor 24, thevoltage divider 2 0 to ground, and through ground to the output resistor6 and back to the diode plate I8,

It will be evident that the picture signal output of the second detectoris impressed across` the resistorf24 of the noise suppressor circuit,and, therefore, across the series combination of diode 4 and resistor 6,since the upper end of this resistor 24 is connected through a conductor`28to the cathode end of the second detector output resistor I6 whilethe lower end of the resistor24 isconnected through a bypass condenser29 and the condenser I3 to the other end of the output resistor I6. Itfollows that the second detector output will cause variations in thecurrent flowing through the limiter diode 4 Whereby there will bevariations in the current flow to the limiter output resistor 6. Sincethe diode 4 has a substantially linear characteristic, the currentvariations through the output resistor 6 produce voltage variationsthereacross which are faithful reproductions of the second detectoroutut. p In Fig. 1 the signal appearing across the limiter outputresistor 6 is applied through a blocking condenser 3| and a seriesresonating coil 32 to the video amplifier 1. It is also applied througha resistor 33, a direct'current ampliiier 30, andgrid condenser 34 to asynchronizing sep- Y arator tube 36.

The input circuit of the video amplier 1 includes a grid leak resistor31. The value of this resistor and the value of the grid condenser 3|are made such that the periodically recurring synchronizing pulsesuwhlchdrive the control grid of the amplifier tube 1 positive periodically,produce a grid leak biasing action such that the direct currentcomponent of the picture is reinserted as described and claimed in thecopending Willans application Serial No. 720,205, filed April 12, 1934,and assigned to Radio Corporation of America.

In addition to employing the previously mentioned peaking and resonatingcoils for holding up the high frequency response, the amplifier tube V1maybe provided with a cathode resistor 38 and a bypass condenser 39,their relative values being such that the amplitude of the lowerfrequency components of the signal is reduced somewhat by degeneration.

The plate of the video amplifier 1 is connected through resonating coils4I and 42 and a protective resistor 43 to the control grid of thecathode ray tube 8. Operating voltage is applied to the plate of thevideol amplifier tube 1 from a suitable voltage divider 44 through aplate resistor 46 and a peaking coil 41, the usual iilter or bypasscondenser 48 being provided in the plate circuit. 'I'he cathode of thecathode ray tube 8 is effectively connected to ground for alternatingcurrent signals by means of a large capacity condenser` 49, while it isdirect current connected, in the-example illustrated, to a suitablevoltage divider 5I connected in parallel with ythe voltage divider 44.

The series resonatingcoll 4I adjacent to the plate of the videoamplifier 1 is preferably shunted by a damping resistor 52. Theprotective resistor 43 is shunted byV a condenser 53v of sufficientcapacity to permit all the frequency components of the signal to Abeapplied to the control grid of the cathode ray tube 8. As described andclaimed in the copending Tolson application Serial No. 315,312, iiledJanuary 24, 1940, and assigned to Radio Corporation of America, the r-ccombination 43-53 functions both to protect the cathode ray tube 8 fromthe eiects of driving its control grid too far positive and to reducedefocusing or blooming of the cathode ray.

'Ihe synchronizing pulse separator 36 is provided for the purpose ofseparating the synchronizing pulses from the picture signals, aftersuitable amplication by an amplifier 30, in accordance with the usualpractice whereby only the horizontal and vertical synchronizing pulsesare supplied to a suitable synchronizing pulse filter indicated at 54for separating the horizontal and vertical synchronizing pulses fromeach other. The vertical synchronizing pulses are supplied to a suitablevertical deflection circuit 56 which causes saw-tooth currentl to flowthrough the vertical deiiecting coils 51, While the horizontalsynchronizing pulses are supplied to a suitable horizontal deflectioncircuit 58 which causes sawtooth current to pass through the horizontaldeflecting coils 59.

The vsynchronizing pulse separator, as shown in Fig. l, may be of thegrid-leak biased type described and claimed in Tolson Patent 2,207,839,issued July 16, 1940. The synchronizing pulse lter 54 may be of a typedisclosed in this Tolson application or any suitable combination ofcapacitors and resistors for effecting frequency discrimination betweenthe horizontal synchro` nizing pulses and the vertical synchronizingpulses which recur at comparatively low frequency and which are ofgreater duration than the horizontal synchronizing pulses.

Referring now to the operation of the noiselimiter circuit, it will beapparent that, so long as the limiter diode 4 is conducting, there willbe a signal voltage appearing across the output resistor 6 if a signalis being supplied from the second detector 5. However, if the signalimpressed upon the cathode I'I of the limiter diode 4 has suiiicientamplitude to raise the cathode I'I to the potential of the diode plateI8, or if it has still greater amplitude whereby the cathode becomesmore positive than the plate, the limiter circuit is effectivelyopen-circuited and no signal will appear across the output resistor 6until the cathode I1 again becomes negative with respect to the plate I8to permit the flow of current through the limiter diode.

The action of the noise-limiter circuit is illustrated in Fig. 1a, wherethe curve 6I represents the current flow through the limiter diode 4plotted against voltage impressed across the diode electrodes I1 and I8.The curves A and B represent the signal impressed across the limiterinput resistor 24 for the conditions of a white" picture and a blackpicture, respectively. Since the synchronizing pulses are applied withpositive polarity to the cathode I'I of the limiter diode 4, any signalssuch as noise signals of the same polarity as the synchronizing pulsesWill raise the cathode I1 to the same potential as that of the diodeplate I8 when their amplitude reaches 17 volts in the specific exampleillustrated. Therefore, as shown by the curves A' and B' in Fig. 1a, anynoise signal in excess of 17 volts (or any other predetermined limit)will not appear across the output resistor 6 of the limiter circuit.

It may be noted that, since the limiter circuit is A.C. coupled to thesecond detector 5 rather than D.C. coupled, a smaller percentage of thenoise will be removed for a black picture than for a white picture, asis evident from a comparison of the curves A and B'.

It will be evident from the foregoing that, by employing the limitercircuit at a point preceding the video amplifier 1 and its associateddirect current reinserting circuit, I have substantially eliminated thepreviously encountered dimculty of noise pulses interfering with theD.C. reinserting action. Such noise interference previously produced onthe control grid on the video ampliiier tube a greater negative biasthan that required for the proper picture background. It will also beapparent that I have avoided a similar difficulty in the operation ofthe synchronizing pulse separator 36. Also, I have substantiallyeliminated the possibility of noise pulses feeding through into thedeflection circuits where they may disturb the deflection circuitoperation.

In Fig. 2, I have shown a noise limiter similar to that previouslydescribed, but which is direct current coupled to the second detectorinstead of A.C. coupled, as in the circuit shown in Fig. 1. In Figs. 1and 2, like parts are indicated by the same reference characters. Inorder to simplify the drawings, the peaking and resonating coils havebeen omitted in Fig. 2.

It will be noted that, in Fig. 2, the plate I8 of the limiter diode 4 isconnected through the limiter output resistor 6 to a positive point on avoltage divider 62, and that the limiter circuit is completed throughthe voltage divider 62 to ground, through groundto the midpoint of thesecondary winding I2 and through the detector output resistor I6 and aconductor 63 to the cathode I1 vof the limiter diode.

The action of this circuit is substantially the same as previouslydescribed, the voltage applied to the plate of the limiter diode vbeingsufficient to maintain a flow of current therethrough until the signalimpressed upon the cathode of the limiter tube exceeds a predeterminedvoltage. If the detector output voltage is in excess of this limitingvoltage, it effectively opens the limiter circuit by making the diodenon-conducting and no signal appears across the output resistor 6 forthe duration of a noise pulse exceeding this voltage limit.

The operation of the circuit of Fig. 2 is illustrated by the curvesinFig. 2a, where the curve 64 represents the limiter-diode characteristicand where the signal impressed upon the limiter circuit is indicated at65, It is assumed that the D.C. component of the picture signal has beentransmitted. It will be apparent that the signal appearing across thelimiter output resistor ,8, represented by the curve 65', containssubstantially no noise having an amplitude in excess of thesynchronizing pulses. The curve 65' also shows that, as a result of therlirect current connection, this is true for all the picture signalregardless of whether it represents a white" picture or a black picture.

Referring to-Fig. 3, the invention is shown applied to` an .automaticvolume control circuit of the synchronizing pulse operated type coveredbroadly by Holmes Patent No. 2,109,618, issued March 1, 1938, andassigned to Radio Corporation of America. In this figure, the limitercircuit is direct current connected and is otherwise substantially thesame as the limiter circuit shown in Fig, 2. However, a battery 66rather than a voltage divider has been indicated as the voltage source.Like parts in Figs. 2 and 3 are indicated by the same referencenumerals.

The automatic volume control circuit, in the example illustrated,comprises a diode 61 connected in series with the parallel combinationof a resistor 68 and a condenser 69, this parallel combination having atime constant such that the diode 6l is caused to function substantiallyas a peak rectifier, tioned Holmes A. V. C. patent, such a peakrectliier circuit, when connected to measure the peaks of thesynchronizing pulses, will produce a voltage which is a measure of thefading or other change in the signal strength of an incoming signal. Acircuit of this character is especially useful in television systemsemploying direct current transmission, since, in such systems, anaverage carrier A. V. C. system will not operate properly because of thefact that the carrier amplitude is being varied by the direct currentcomponent. It may be noted that in the Holmes patent the A. V. C.peakrectiiler is a triode. The D.C. connected A. V. C. diode circuit isdescribed and claimed in my copending application Serial No. 210,614,iiled May 28, 1938, and assigned to the Radio Corporation of America.

It has been found, however, that a peak rectier type of A. V. C. system,such as shown in Fig. 3, may not operate properly in' noisy locationsunless suitable precautions are taken, because large amplitude noisepulses may control the A. V. C. action and thus reduce the gain of thereceiver at times when the gain should not be reduced. However, i ftheA. V. C. circuit is preceded by my limiter circuit, as shown 'in Fig.3, the A. V. C. voltage which appears across the resistor-conden- Asexplained in the above-menser combination 68-69 will be a true measureof the height of the synchronizing pulses and, therefore, of the amountof gain desired in the receiver. This A. V. C. voltage is applied in theusual manner to a direct current amplifier 1| and from this amplifierthrough suitable filter means to the control grids of the IF amplifiertubes.

Referring to Fig. 4, my limiter circuit is shown applied to a televisionreceiver for the purpose of making all synchronizing pulses oi the sameamplitude before they are impressed upon the deflecting circuits.. Inthe circuit shown in Fig. 4, a synchronizing pulse separator circuitcomprising a diode 12 is connected to the output of the second detector13 of a television receiver. In the example illustrated, the seconddetector comprises a diode 14 and the usual output resistor 16 shuntedby a condenser 11.

'I'he output of the second detector is impressed across the electrodesof the separator diode 12 through a condenser 18 and through a separatoroutput resistor 19. There is also provided a comparatively highresistance resistor 8| connected between the plate of the diode 12 andground. The positive synchronizing pulses which are applied through thecondenser 18 to the plate of the diode 12 cause a biasing voltage to bemaintained across the -resistor 8|, this bias being of such value thatonly the synchronizing pulses cause a ow of diode current whereby onlythe synchronizing pulses appear across the output resistor 19.

Since, for various reasons, these synchronizing pulses may beof unevenamplitude at this point, it has been found desirable to level them; thatis, make them all of like amplitude by means of a limiter` circuitsimilar to that previously described. As applied in Fig. 4, lthisleveling or limiting circuit comprises a limiter diode 82 and a resistor83 which are connected across the output resistor 19 of the separatingcircuit and in series with a source of D.C. voltage for maintainingnormally a ow of current through the limiter diode.

In this application of the invention, the D.C. voltage is set at such avalue that each synchronizing pulse drives the cathode of the limiterdiode 82 to a potential at least-a:` great as the potential of the plateof the diode 82. As a result, the tops of all synchronizing pulses areclipped H to the same level, this clipping or leveling action beingillustrated in Fig. 4a, wherc the limiter diode characteristic isrepresented at 84. The synchronizing pulses appearing acrcss theseparator output resistor 19 are indicated by the curve 86, while theleveled synchronizing pulses appearing across the limiter outputresistor are indicated by the curve 96'.

In Fig. 5, I have shown a leveling circuit for synchronizing pulses,which is similar to that shown in Fig. 4, like parts in Figs. 4 and 5being indicated by the same reference characters. The circuits of Figs.4 and 5 differ in that the separating circuit is connected to differentpoints in the receiver in the two figures. In Fig. 5, theseparator tube12 is connected to the output circuit of a video amplifier -81 ratherthan to the output of the second detector, asin Fig. 4.

In Fig. 6, there is shownv another circuit in which my invention isutilized for leveling synchronizing pulses, where the limiting circuitis shown applied to the output -circuit of a synchronizing pulseseparator tube 88 of the type illustrated in Fig. 1. The limiter diode89 and its output resistor 9| are connected across the plate resistor 92of the separator tube, a suitable 75 positive voltage being applied tothe plate of the limiter tube 89 by means of a battery 93 whereby thediode 89is conducting until the signal impressed upon its plate exceedsa predetermined limit.

In Fig. 7, the invention is shown applied to a television receiver insuch a way as to function as a noise limiter for the synchronizingchannel. In this figure and in Fig. l, like parts are indicated by thesame reference numerals. In this particular application of theinvention, the noise-limiter diode 94 and its output resistor 98 areconnected across the plate resistor 91 of the video amplifier 1 with theplate of the diode 94 maintained normally at a certain D.C. potential.It will be noted that a substantially noise-free signal is applied tothe synchronizing separator tube, as in the case of Fig. 1, whereby thesynchronizing action of the receiver is improved.

In Figs. 8 and 9, there are shown embodiments of the invention in whichthe current flowing through the limiter diode is prevented from flowingthrough the output resistor of the second detector, even though thelimiter circuit is direct current connected to the detector.

'In Fig. 8, where parts corresponding to those in Fig. 1 are indicatedby like reference numerals, this is accomplished by means of a balancedcircuit, a resistor |0| which may have the same resistance as that ofthe limiter output resistor |02 being connected between the cathode ofthe limiter diode |03 and a point onv the voltage divider |04 such thatany ow of current through the resistor I 0| and the ouput resistor I8 ofthe second detector balances out any ilow of limiter diode currentthrough the detector output resistor I6.

In Fig. 9, the current flow of the limiter diode |05 through the outputresistor |06 of the second detector |01 is prevented by means of abattery |08 which is connected in the detector circuit to oppose theflow of such current. By means of arrangements, such as shown in Figs. 8and 9, it is possible to avoid having the'quality of the detectoroutputA impaired by the'flow of an undesired direct current componentthrough the detector o utput resistor. The circuits of Figures 8 and 9are described and claimed in copending application Serial No. 436,559,led March 28, 1942, in the name of Ralph S. Holmes.

In the drawings, the values of certain circuit elements have beenindicated, by way of example, in ohms, megohms, and microfarads. Also,in certainiigures, the voltages at different parts of the circuit havebeen indicated by way of example.

From the foregoing description, it will be apparent that variousmodications may be made in my inventio'n without departing from thespirit and scope; thereof.

What I claim as my invention:

1. A television receiver for the reception of a carrier wave negativelymodulated by a composite signal consisting of picture signals 'andsynchronizing pulses and also modulated by the direct current componentof the transmitted picture, said receiver comprising a diode detectorfor demodulating said received signal to produce picture signalsandsynchronizing pulses, said detector including an output resistor, and anamplitude limiter circuit comprising a diode, an output resistor and asource of direct-current potential connected in series with each otherand direct current connected across .the output resistor of saiddetector, said diode being connected in such 'a direction in the circuitthat the synchronizing pulses appearing across the detector outputresistor and supplied to the diode through said direct currentconnection oppose current ow through the diode, and said source ofdirect-current potential being connected in the circuit with the properpolarity to maintain a ow of current through the diode so long as thesignal appearing across the detector output resistor of the polarity ofthe synchronizing pulses vdoes not exceed a predeterminedamplitude, the

resistance of the limiter output resistor being at least several timesthe resistance of the detector output resistor.

2. In a television system of the type in which the direct currentcomponent of the picture is transmitted whereby the carrier varies inaccordance with the changes in background of the transmitted picture, areceiver comprising a diode detector for demodulating said transmittedpicture signals, said detector having an output resistor, a utilizationcircuit, and a signal amplitude limiter through which the output of saiddetector is fed to said utilization circuit, said limiter comprising adiode and an output resistor connected in series, means for inserting adirect current voltage in series with said series combination of diodeand output resistor and with the proper polarity to maintain a ow ofcurrent through said diode until the signal to be limited exceeds apredetermined amplitude, said direct current voltage having a valuewhich is independent of the average carrier value of said transmittedsignal, and means including said direct current connection for applyingsaid signal across the series combination of diode, output resistor andD.-C. voltage with such polarity 1 picture signals and synchronizingpulses, said detector including an output resistor, and an amplitudelimiter circuit comprising a diode, an output resistor and means forproviding a direct current voltage in series with said diode and saidoutput resistor, said series combination of diode, output resistor anddirect current means being direct current connected across the outputresistor of said detector, said diode being connected in such adirection in the circuit that the synchronizing pulses appearing acrossthe detector output resistor and applied to the diode through saiddirect current connection oppose current ow through the diode, and saiddirect current voltage having the proper polarity to maintain a ow ofcurrent through the diode so long as the signal appearing across thedetector output resistor having the polarity of the synchronizing pulsesdoes not exceed a predetermined polarity, the resistance of the limiteroutputl resistor being at least several times the resistance of thedetector output resistor.

4. In a television system of the type in which the direct currentcomponent of the picture is transmitted whereby the carrier varies inaccordance with the changes in `background of the transmitted picture, areceiver comprising a diode detector for demodulating said transmittedpicture signals,` said detector including an output resistor, autilization circuit, and a signal amplitude limiter through which theoutput of said dectector is fed to said utilization circuit, saidlimiter being direct current connected to said detector and comprising adiode and an output resistor connected in series, means forV inserting adirect current voltage in series with said series combination of diodeand output resistor and with the proper polarity to maintain a ow ofcurrent through said diode until the signal to be limited exceeds .apredetermined amplitude, and means including said direct currentconnection for applying said signal across the series combination ofdiode, output resistor and D.C. voltage with such polarity as to opposecurrent ow through said diode, the resistance of the limiter outputresistor being at least several times the resistance of thedetectoroutput resistor.

CIRO C. MARTINELLI.

